1. Field of the Invention
The present invention is concerned with semiconductor bridge igniters, which are useful in initiating the detonation of explosives. In particular, the present invention is concerned with semiconductor bridge devices employing multilayer metallized lands and/or a multilayer metallized bridge, which devices provide greatly improved performance characteristics as compared to prior art devices, and with a method of making the same.
2. Related Art
U.S. Pat. No. 4,708,060 of R. W. Bickes, Jr. et al, entitled "Semiconductor Bridge (SCB) Igniter", issued on Nov. 24, 1987, discloses a structure comprising a semiconductor or other suitable "electrical material" supported upon a non-electrically-conducting substrate and having metallized lands formed thereon. The electrical material must, according to the Bickes et al patent, (column 3, line 41 et seq.) develop a temperature coefficient of electrical resistivity which is negative at some temperature, for example, some temperature above room temperature, such as about 100.degree. C. Bickes et al teaches (column 3, line 19 et seq.) that the precise temperature is not critical and that essentially all semiconductors will have this property at sufficiently high doping levels, as will some other materials, such as rare earth metal oxides (column 3, line 54 et seq.). Preferred doping levels for semiconductors are preferably essentially at or near the saturation level, for example, approximately 10.sup.19 atoms per cubic centimeter. A typical doping component would be phosphorus atoms used for doping n-type silicon. Lower doping levels may also be used under appropriate conditions according to Bickes et al, for example, doping levels lower by a factor of 2 from the above-stated saturation levels are stated to be adequate and to provide corresponding resistivity values on the order of 10.sup.-3 to 10.sup.-4, for example, about 8.times.10.sup.-4 ohm-centimeters.
Bickes et al discloses providing the semiconductor or other "electrical material" in the form of two relatively large surface area pads connected by a small surface area bridge, the pads being covered by metallized lands which leave the bridge exposed (see FIG. 1A and column 2, lines 40-52). Such devices are referred to as semiconductor bridge devices and the metallized lands provide electrical contacts for connecting a semiconductor bridge device in a circuit by soldering or the like. Bickes et al disclose (column 4, lines 35-46) that such metallized coatings will be composed of highly electrically conductive metals such as gold, silver, copper, aluminum, etc. The semiconductor bridge device of Example 1 of Bickes et al employs aluminum lands.
Such semiconductor bridge devices are stated to have the requisite characteristics for initiating an explosive maintained in contact with the semiconductor. As stated at column 2, lines 53-61 of Bickes et al, initiation of the explosive is believed to be caused by a combination of ignition and initiation effects, essentially a process of burning but also involving the formation of a thin plasma and a resultant convective shock effect.